1. Field of the Invention
The present invention relates to a piercing apparatus which is suitable for the formation of minute holes in a web material such as a tip paper used to connect a cigarette with a filter plug.
2. Related Art
Recently, a tip paper of a type formed with minute holes which serve to adjust the air permeability of the tip paper is employed for the production of filter cigarettes. A piercing apparatus for forming such minute holes in a tip paper is known from Japanese Provisional Patent Publication No. 5-138381, for instance. This piercing apparatus is configured to obtain a deflected laser beam, by deflecting, with use of a polygon mirror, a laser beam continuously supplied from a laser source and to irradiate the deflected laser beam via an optical system onto a tip paper which travels at a constant speed, thereby forming minute holes at regular intervals in the tip paper.
In the aforementioned piercing apparatus, the spacial interval at which the minute holes are formed is naturally determined based on a relationship between the traveling speed of the tip paper and the rotational speed of the polygon mirror. Hence, it is difficult to immediately adapt the piercing apparatus to a changeover of kinds of tip papers which necessarily follows the changing of a kind or brand of filter cigarettes to be manufactured by the cigarette manufacturing machine.
Specifically, a required spacing with which minute holes are to be formed in the tip paper varies in dependence on the brand of filter cigarette. In a case where minute holes are formed in multiple rows, moreover, a required number of rows varies depending on the brand of cigarette. To satisfy the requirements in respect of the number of rows or the spacing of minute holes, the arrangement of the optical system, the tip paper traveling speed, or the polygon mirror rotational speed must be changed in a piercing apparatus of the aforementioned type. In the case of changing the polygon mirror rotational speed, however, there occurs a substantial change in the intensity of the deflected laser beam irradiated onto the tip paper, so that the size of minute holes changes. Similarly, the hole size changes, if the tip paper traveling speed is changed. Since the size of minute holes formed in a tip paper greatly affects the air permeability of the tip paper, difficulties will be encountered in manufacturing filter cigarette having a required air permeability, if the hole size is deviated from a desired one.
Conventionally, therefore, a measurement of the actual air permeability of a tip paper is generally carried out, and on the basis of the measured air permeability, the quality of minute holes formed in the tip paper is evaluated. Further, based on the evaluation result, the output intensity of the deflected laser beam or the tip paper traveling speed, for instance, is adjusted, so as to obtain a tip paper having a desired air permeability.
However, such an air permeability measurement and a subsequent evaluation must be made offline and take a lot of time. This makes it impossible to immediately carry out adjustments to be effected on the side of the piercing apparatus or the cigarette manufacturing machine for correction of the air permeability. After all, it is difficult to stably carry out the formation of minute holes having a required size in a tip paper with a required spacing, while adjusting the size of or the spacing between minute holes, by using the aforementioned piercing apparatus in combination with an air permeability measuring device.